1. Field of the Invention
The present invention relates to the field of signal processing, and in particular, relates to the field of compressing the information contained in a signal into significantly smaller bandwidths than are required by conventional systems without loss of subjective quality. More particularly, the invention relates to a field of electronic signal processing which depends on the encoding of only the times of occurrence of maxima and minima, i.e., the extrema, of an analog waveform for the transfer of intelligible and high quality communications without the encoding of the amplitude values of the extrema themselves. The invention presents a novel model of the human sensory system, and particularly the auditory system. The invention has particular application in human sensory aid and simulation systems, speech processing, in bandwidth and data compression, in noise reduction systems and in subjective bandwidth extension.
2. Description of the Prior Art
Prior art signal amplification techniques make it possible to raise intensity levels of signals obtained from various sources such as transducers and devices that detect modulated signals. By amplifying the amplitude, signals become compatible with other devices such as recording, transmission and reproduction systems. Amplifiers find a large number of applications such as sound and video systems, hearing aids and sensory simulation systems. In the latter case, a lack of input dynamic range becomes noticeable since the human auditory system functions for signals having a dynamic range of 120 dB or more while electric amplifiers barely reach 100 dB. This effect causes distortion and/or insensitivity. Generally, dynamic range of a signal processing device is limited at one extreme by the signal to noise (S/N) ratio, and at the other extreme, by the power supply voltage. Thus, any system with essentially infinite dynamic range (&gt;200 dB) must be able to process signals without being affected by one of these limitations. For example, a system in which information is converted into binary form, independent of the amplitude values of the analog waveform will satisfy this criteria. It has generally proved difficult, however, to maintain dynamic range while at the same time decreasing the bandwidth required for accurate frequency domain signal reproduction.
Methods for compressing the bandwidths required for information transfer are known. For example, delta modulation in which the differences between amplitudes of successive samples are encoded, offers bandwidth savings. Other systems include adaptive delta modulation, adaptive transform coding, vocal tract modeling and linear predictive coding. Although offering substantial bandwidth reductions, those systems suffer from complexity and in the case of the latter two systems, inability to transfer other than speech signals.
Methods for encoding only the characteristics relating to the extrema of an analog signal are known. In particular, techniques for determining the times of occurrence of extrema have been devised in the past. The process of detecting and encoding the times of occurrence of extrema is usually achieved by a signal processing circuit which comprises means for differentiating an input signal to shift minima and maxima to zero crossings, and an infinite clipper having a fast response time with respect to the bandwidth of the input signal for clipping the differentiated signal in order to detect these zero crossings.
In the early paper by Licklider and Pollack, "Effects of Differentiation, Integration, and Infinite Peak Clipping upon the Intelligibility of Speech," Journal of the Acoustical Society of America 20, 42-51 (1947), the intelligibility of distorted speech waveforms was investigated. It was discovered that intelligible, though poor quality speech could be obtained by the process of first differentiating the speech waveform followed by infinite peak clipping. Although this technique does solve the problem of input dynamic range, it was found that the performance is strongly degraded by background noise. Additionally, although the clipped signals are intelligible, quality is poor due to the information lost in the clipping stage of the process.
Another research effort, that by Thomas and Sparks, "Descrimination of Filtered/Clipped Speech by Hearing Impaired Subjects", Journal of the Acoustical Society of America 49, 1881-1887 (1971), investigated the intelligibility to hearing impaired persons of speech waveforms that had first been high pass filtered before clipping. It was found that intelligibility could be increased for some hearing impaired persons using this process. Again, however, this method suffers from the same problems noted by Licklider and Pollack.
In British Patent Specification Nos. 1501874 and 843607, techniques for measuring the times of occurrence of the maxima and minima are shown. In these systems, however, the amplitudes of the waveforms at the maximum and minimum points are also sampled and processed in addition to determining the times of occurrence of the signal extrema.